1. Field of the Invention
The present invention relates to an organopolysiloxane composition curable into elastomeric state, even in a confined atmosphere, said composition comprising a crosslinking agent bearing acyloxy radicals bonded to silicon atoms, as well as an effective amount of at least one CaO, SrO and/or BaO cure accelerator.
2. Descriotion of the Prior Art
Organopolysiloxane compositions of the aforementioned accelerated-cure type are known to this art, and are described, e.g., in French Patent No. 1,193,271, where the accelerator is zinc oxide, in British Patent No. 1,308,985, where the accelerator is a zeolite, and in European Patents Nos. EP-A-118,325 and EP-A-117,772, where the accelerator is selected, respectively, from an alkali and alkaline earth metal hydroxide, or from a mixture of water and an alkali or alkaline earth metal phosphate and/or polyphosphate.
In contrast to the single-component compositions containing acyloxy radicals but no accelerator (described, for example, in French Patent Nos. 1,198,749, 1,220,348 and 2,429,811 and European Patent No. EP-A-102,268), these accelerated-cure compositions are generally not stable in storage, but their crosslinking time is much shorter, for example, on the order of a few minutes to 60 minutes.
The accelerated-cure compositions, where this cure (or this crosslinking) is independent of the degree of humidity of the surrounding atmosphere, are employed in fields of application where this property is particularly advantageous, such as, for example, the production of "in situ" seals in the motor vehicle industry.
Furthermore, in single-component compositions including acyloxylated crosslinking agents, it is known, from U.S. Pat. No. 3,061,575, to add at least 1.5 mole of MgO per mole of organotriacyloxysilane to improve the heat stability of the crosslinked elastomer, especially if the latter is in a confined atmosphere.
For industrial applications involving continuous seal deposition, automatic apparatus is typically used for depositing a two-component diorganopolysiloxane composition, which includes a mixing head fitted with an application nozzle, the latter following the outline of the seals to be produced.
To prevent solidifying of the composition in the mixing head, which is maintained at ambient temperature when it is in use or during stoppages on a continuous seal application line, it would be desirable to have available an accelerated-cure composition having, on the one hand, very slow crosslinking at ambient temperature (longer than at least about 10 minutes) after the two components are mixed, namely, after mixing of the cure accelerator and, on the other hand, a very fast crosslinking of the seal after its formation.
It would also be desirable, furthermore, to have available a composition, the time of crosslinking of which at ambient temperature and under the influence of heat could be reduced or increased at will, enabling parameters other than temperature alone to be employed.
Complete control of the crosslinking time at ambient temperature would make it possible to shut down a continuous seal application line without the necessity fo purging the mixing head. A fairly slow cure at ambient temperature would also promote the production of suitable adhesion of the seal to the substrates, because the composition would spread easily over such substrates with a sufficiently long contact time to ensure suitable wetting of the surface, because the adhesion of the seals to the substrates is frequently an essential property of the seal.
It is also desirable to have available a composition which cures rapidly under the influence of a temperature rise which is as small as possible. In fact, a moderate rise in temperature would make it possible to maintain high production rates on industrial lines for continuous seal application.
Where the elastomer produced is concerned, a composition of this type should additionally have good mechanical properties, in particular when hot in a confined atmosphere in respect of the residual compression deformation (RCD).
Known accelerated-cure compositions do not possess adequate combination of the aforesaid desirable properties. In fact, accelerated-cure compositions containing zinc oxide and zeolites do not possess, in particular, mechanical properties which are satisfactory when hot in a confined atmosphere.
The compositions described in European Patent Nos. EP-A-118,325 and EP-A-117,772 have enabled a major step forward to be made in respect of the retention of mechanical properties, when hot, in a confined atmosphere, and have made it possible to employ compositions for the adhesive bonding or sealing of components travelling on industrial assembly lines, or for which there are no available storage areas which would ensure their complete cure. Nevertheless, these compositions may have a setting time at ambient temperature which is too short and their adhesion to substrates, especially metallic substrates, may be inadequate.